1. Field of the Invention
The invention relates to novel polyesters and a method of their production by microwave heating.
2. Description of the Prior Art
Numerous issues such as price uncertainty, manufacturing costs, environmental disposal, and geo-political issues affect the petroleum-based polymer industry. As an alternative, there is increasing interest in the development of biodegradable non-petroleum-based polymers which has risen steadily since the 1980s (Pramanick and Ray. Polym Bull 1988, 9:365; Pachauri and He. ASABE Mtg 2006, #066223, 1; Holser. J Appl Polm Sci 2008, 110:1498; and Budhavaram and Barone. ASABE Mtg 2008, #083836, 1). However, the price of the traditionally-used non-petroleum based polymers (e.g., poly(lactic acid), PLA) is high and not competitive with petroleum-based polymers (e.g., polyethylene terephthalate, PET) (Budhavaram and Barone, ibid). Therefore, the development of lower cost non-petroleum-based polymers is needed. Copolyesters can be prepared using polyfunctional acids and alcohols such as citric acid and glycerol, respectively (Pramanick and Ray. ibid; Pachauri and He. ibid; Holser. ibid; and Budhavaram and Barone. ibid).
Citric acid is relatively inexpensive and is the most commonly produced organic chemical obtained by fermentation (Soccol et al. Food Technol Biotechnol 2006, 44:141; and Budhavaram and Barone, ibid). Citric acid is a weak organic acid that naturally occurs in a variety of vegetables and fruits (Soccol et al. ibid). In 2007, approximately 1,700,000 million tons of citric acid was produced with as much as 50% produced in the People's Republic of China. About 50% of the citric acid is used in beverages, 20% in food applications, 20% in detergent applications and 10% in non-food related applications such as cosmetics, pharmaceutical and chemical industrial products (Soccol et al. ibid).
The production of biodiesel has resulted in an enormous excess of glycerol (Budhavaram and Barone. ibid; Yazdani and Gonzalez. Curr Opin Biotechnol 2007, 18:213; Lines. Published online: 3 Mar. 2009, DOI: http://snrecmitigation.wordpress.com/2009/04/19/an-exploding-market-utilizing-waste-glycerol-from-the-biodiesel-production-process; and Moon et al. Appl. Biochem. Biotechnol 2010, 161:1). Biodiesel is commonly produced from the transesterification of vegetable oil or animal fat feedstocks with glycerol generated as a co-product. For every ton (i.e., 1,000 Kg) of biodiesel manufactured, 100 Kg of glycerol is produced (Budhavaram and Barone. ibid; Yazdani and Gonzalez. ibid; Lines. ibid; and Moon et al. ibid). The 2000 market price for glycerol has plummeted to only 1/10th of that price by 2009 (Pramanick and Ray. ibid; Lines. ibid; and Moon et al. ibid). We can presume in the future that the price of glycerol will continue to remain low and perhaps even continue to decline as biodiesel production escalates (Pramanick and Ray. ibid; and Lines. ibid). Industrial producers of glycerol are actively seeking new uses for this excess glycerol in order to increase the value for this product (Pramanick and Ray. ibid; and Ebert. J Biodies Mag 2007, 4:1). Several investigators have sought to employ glycerol as a feedstock for microorganisms in order to produce high-value chemical products such as succinic acid, propionic acid, ethanol, butanol and hydrogen terephthaleate (Moon et al. ibid).
Normally, citric acid and glycerol are considered plasticizers and are employed to increase the fluidity of the material to which they are added (e.g., plastics, concrete, wallboard, and clay). Glycerol may be combined with tricarboxylic acids such as citric acid to form a corresponding ester (Pramanick and Ray. ibid; Pachauri and He. ibid; Holser. ibid; and Budhavaram and Barone. ibid). Copolymers of glycerol and citric acid are relatively benign and biodegradable (Holser. ibid; and Budhavaram and Barone. ibid). The importance of heating to achieve glycerol citric acid copolyester polymerization has been previously demonstrated (Pramanick and Ray. ibid; and Budhavaram and Barone. ibid). Budhavaram and Barone (ibid) prepared copolyesters by blending various molar ratios of glycerol and citric acid to obtain a 15 ml solution in a beaker coupled with 9 ml of distilled water then stirring for 10 minutes while heating at 25-140° C. to drive off the water of reaction. Subsequently, the reaction continued for 2 to 3 days in order to achieve polymerization at 75° C. employing 1.2:1 molar ratio of citric acid:glycerol blend and even lower polymerization levels were achieved by heating at 45° C. even after 6 days (Budhavaram and Barone. ibid). Pramanick and Ray (ibid) mixed glycerol and citric acid in various molar ratios with p-toluene sulfonic acid within a flask and then incubated at 170° C. under nitrogen for 12 hr. After the elimination of the water by-product, the blend (i.e., glycerol citrate copolyester) was further heated for an additional 4 hours under the same temperature and conditions. A solid amorphous copolyester was collected from the reaction vessel which was purified by leaching several times with boiling ethanol (Pramanick and Ray. ibid). This amorphous solid copolyester was insoluble in water and other common organic solvents suggesting this polymer was “sufficiently cross linked” (Pramanick and Ray. ibid).